Biomaterials are used in numerous medical applications today, such as fixation devices, replacements and surgical equipment. Implants are typical examples of a biomaterial application and there are several different implant materials used today. Many of these are however designed to stay in the body permanently even though they only serve their function temporarily. Even if the materials are biocompatible there are several complications associated with long term presence of implants, including allergy and sensitization. Many of these implants are only left in the body to eliminate risks concerning the removal process. Removing an implant usually involves surgery which increases both cost and patient morbidity. These negative consequences would be eliminated by using a biodegradable material. A completely biodegradable implant would dissolve and be absorbed by the body after the healing process is completed. Commonly used metallic implant materials include stainless steels, titanium alloys and cobalt-chromium alloys. These materials have great mechanical properties and are often used in load bearing applications. The mechanical properties of some common alloys can be seen in Table 1. However, many metallic corrosion products are harmful to the body and none of the implant metals used are biodegradable. Ceramic materials are known for their high strength and are generally biocompatible. Synthetic hydroxyapatite and other calcium phosphates as well as bioactive glass are commonly used materials for bone augmentation and bone replacement. They resemble the bone structure which gives good chemical bonding to bone and is therefore defined as bioactive. Alumina and zirconia are commonly used inert biomaterials. Ceramic coatings are frequently used on metallic implants to increase the biocompatibility and to induce bone ingrowth. The biggest disadvantage of ceramics is high brittleness, as can be seen in Table 1. There are numerous polymeric biomaterials used today, such as polyethylene (PE), polyvinylchloride (PVC), poly(methyl methacrylate) (PMMA) etcetera. However, all polymers have the disadvantage of low strength which eliminates their possibility to be used in load bearing applications, such as for example bone fixation devices.
TABLE 1Mechanical properties of magnesium, human bone and somecommonly used biomaterials.ElasticTensilemodulus Density Yield strengthstrength (GPa)(g/cm3)(MPa)(MPa)Magnesium 4511.741 702 1762Human cortical  5-2331.8-2.03106-2244 51-1724bone(compressive)Stainless steel19058.03300-12005480-6203TI6Al4V11414.431896110001Alumina38043.9562260-26006 2704Bioactive glass 353—— 40-2002,3Synthetic 73-11773.176007  0.77hydroxyapatite(compressive)Biodegradable 12.881.59—339-3949PGABiodegradable1.2-34—— 28-484L-PLAThe ranges of values are depending on testing conditions or anatomical location.References are compiled from different sources;1(ASM-International 1999),2(Cardarelli 2008),3(Witte, Hort et al. 2008),4(Kutz 2002),5(Bartel, Davy et al. 2006),6(Harper 2001),7(Staiger, Pietak et al. 2006),8(Maurus and Kaeding 2004),9(Brandrup, Immergut et al. 2005).